Ricardo Montalban was in ridiculously good shape throughout most of his life. Hell, he still managed to look well even when he became wheelchair dependent!
IIRC, Montalban's background was in musical theatre and dance, so it's not a stretch to say that he continued the vigorous training regimes of those branches when he became a film and television actor, perhaps out of habit. There have been many 60+ year old actors like Harrison Ford and Sean Connery for example. who remain in great physical shape. The Montalban "chest" conspiracy has always sounded pretty odd.
As for a "CGI Montalban", it could be done for a brief cameo but not for a whole movie. To pull it off, they'd need to find a actor or stunt man, who is similar to Montalban in stature along with re-using footage and audio from "Space Seed", maybe even from TWOK too. The best result would probably be similar to the T-800 models (CGI is used to create Arnold Schwarzenegger's likeness) seen in Terminator Salvation. They can't do what they did with Jeff Bridges (real-time motion capture and CGI) because Montalban is no longer alive, sadly.